The uninterrupted power supply (UPS) finds its more and more extensive application nowadays, especially in a system requiring a stable power source, e.g. a computer room in order not to interrupt a task under execution. As an example, for a computer encountering an interrupted power source, if the interrupted task is just beginning to run, we just need to reexecute the task. Nevertheless, if a heavy task is involved in and is nearly half performed, no one could then doubt the importance of the UPS.
As shown in FIG. 1 showing a prior UPS system, when the power source is normal, an AC/DC converter 1 and a DC/AC converter 2 will respectively provide DC and AC outputs, and the power source will charge through a charger 5 a power storing device 3. Thus, when the power source becomes interrupted, power storing device 3 will discharge in order to provide DC and AC outputs respectively through a DC/DC converting device 4 and DC/AC converter 2.
Such UPS system suffers from some disadvantages. Firstly, charger 5 is required for charging power storing device 3. Charger 5, however, has a cost not less than that of an electric converting device and includes a plurality of switching circuits which will generate many harmonic waves in operation and will in turn adversely affect the power source quality, e.g. the power factor (which could be overcome by introducing a power factor compensator which, nevertheless, is not cost-effective.)
The larger the capacity of the UPS is, the bigger the charger will be which means the more the cost thereof will be. In addition, there is a safety problem in that there is no galvanic isolation between the input and output ends.
It is therefore attempted to obviate the provision of the charger in order to solve the problems above-described.